Over the years there have been many attempts to improve the quality of reproduction of recorded sound, particularly music. The bulk of this work has been directed to improvements in the signal-to-noise ratio of sound recordings and reproduction equipment, reduction of "wow and flutter" in mechanical reproduction systems, and the like. There has also been substantial work directed toward enhancement of the sound of audio recordings. Specifically, it has been an object of numerous inventors over the years to enhance reproduced audio in order to provide a more "live" sound in reproduced music, to improve the listening experience in an aesthetically pleasing way.
International Patent Application No. PCT/US91/09375 in the name of Eldon Byrd provides an extensive discussion of various systems designed to thus enhance the aesthetic qualities of reproduced audio. Byrd claims a system having first and second signal processing stages wherein additional harmonic frequencies related to fundamental frequencies in the audio signal are phase shifted as a function of frequency, such that the phase shift increases in a negative direction with increasing frequency, that is, so that the higher frequency signals lag the lower frequency signals.
Additional documents relating to devices for improving the aesthetic qualities of reproduced music include U.S. Pat. No. 4,841,572 to Klayman, broadly disclosing a system for simulating stereo sound in a monaural system. Klayman teaches splitting the input signal and delaying one portion thereof by a 90.degree. phase separation, so that one of the two signals is time delayed by an amount varying with the frequency of each portion of the delayed signal. Klayman suggests that only signal information below about 10 KHz is relevant to operation of the system.
Another system of interest is shown in Tominari U.S. Pat. No. 4,873,722, wherein an audio signal is supplied to loudspeakers in the normal way together with a second signal comprising the main channel signal having been phase delayed according to a frequency versus delay time characteristic such that the lower frequencies are delayed more than the higher frequency components.
Another prior art system for increasing the aesthetic qualities of recorded sound is shown in U.S. Pat. No. 3,924,072 to Turner et al, wherein a monaural signal is played through one of a pair of speakers while the same signal is phase-inverted and played through the second speaker of the pair, so as to provide audible information responsive to the phase difference between the two channels.
It is also known to play the main audio signal through front and rear pairs of speakers in a listening room, wherein the signal played through the rear pair of speakers is time-delayed so as to simulate an echo of the signal from the rear of a concert hall or other listening room. Systems are known which provide variable time delay to simulate listening rooms of various sizes, and also in which the reflectivity of various types of listening rooms are effectively simulated.
Further generally relevant disclosures are found in U.S. Pat. No. 3,983,334 to Takahashi et al; 4,495,637 to Bruney; 4,887,045 to Nakayama; 4,567,607 to Bruney et al; 4,831,652 to Anderson et al; 4,817,162 to Kihara; 4,706,287 to Blackmer et al; and 4,908,858 to Ohno.
The prior art has also failed to provide circuitry in a sufficiently convenient and inexpensive form to permit the same circuit components to be used for audio enhancement in a number of different applications.